1. Field of the Invention
This invention relates to an electrophotographic image forming apparatus, and more particularly to a cleaning device for removing residual toner and debris from the surface of a biased transfer roll (BTR).
2. Description of Related Art
Typical cleaning methods in electrophotographic applications such as xerography, include wiping with a fur brush, a web, a blade and the like, a method using magnetism or a magnetic brush, a method using an air flow and/or a combination of at least several of the above. Cleaning methods employing an air flow are essential for use with the apparatus described herein.
Turning now to FIGS. 1 and 2, cleaning apparatuses known in the art are depicted which include at least some combination of an air flow, a BTR and a brush cleaner which may be electrostatically charged. In FIG. 1, an electrostatic brush cleaner 1 is depicted, including a cleaner housing 43 with upstream and downstream air inlets 40. To be effective, electrostatic brush cleaners must balance the air flows from the two sides of the housing 43. This is typically done by controlling the cleaner housing 43 spacing to the photoreceptor 16, the spacing between the brush 41 and the cleaner housing inner wall 35 and/or by adding interferences 39 between the brush 41 and the cleaner housing inner wall 35 near an air inlet 40 to create a pressure situation which will affect the air flow (air flow direction indicated at arrow 11). Additionally, positively and negatively charged detoning rolls 20a, 20b are used to assist the air flow in removing particles from the electrostatic brush 41. A flicker bar 37 is provided to help knock the toner particles free of the electrostatic brush 41 fibers.
In FIG. 2, a BTR cleaner 2 is depicted, using a rotating brush 3 in combination with an air system. The brush 3 removes toner from the BTR 20 and the air flow detones the brush fibers, with air flow direction indicated at arrow 11.
The main disadvantages of the prior art devices discussed above include large size, insufficient component life, BTR surface abrasion and high unit manufacturing costs.
Cleaning apparatuses employing an electrical bias to clean residual toner from an electrostatically charged surface in an electrophotographic device are also known.
U.S. Pat. No. 4,647,186, to Armstrong et al., discloses an apparatus for scavenging undesired charge particles from the surface of a recording element. The apparatus consists of a grid structure comprising a plurality of parallel, non-magnetic, electrically conductive wires. The plate is connected to an AC/DC power supply whose polarity is opposite to that of the charge particles to be scavenged. The AC grid bias functions to alternately attract the charged particles from the recording element and towards the grid, and then repel such particles from the grid itself. The grid, composed of a plurality of wires allows the scavenged particles to pass (or be pulled) through the grid by the magnetic influences of a magnetic brush applicator positioned directly beneath the grid.
U.S. Pat. No. 4,530,595, to Itaya et al., discloses a method and apparatus for cleaning the surface of an electrostatic image holder, where a DC voltage and/or an AC voltage whose polarity is opposite to that of the residual developer on the electrostatic image holder is impressed between an electrostatic image holder and a film member, one side of which is electrically conductive and the other side is insulated. The insulated surface of the film member is held close to the image holder and a voltage is impressed between an electrode and an electrostatic image holder. A removing means is used to remove the developer adhered on the insulating surface of the film member.
U.S. Pat. No. 4,479,709, to Syukuri et al., discloses a cleaning method to remove toner attached to an image retaining member of an electrophotographic copying machine, without damaging the surface of the image retaining member. An alternating electric field is applied to the surface to be charged. When applied, the toner on the photosensitive receptor drum moves to the roller and adheres thereto. The toner on the surface roller is scraped off by a blade and is collected in a recovery box.
It is important, for purposes of this invention, to clearly describe the BTR function in the electrophotographic apparatus. Paper, to which the image will be transferred, is fed into a nip formed by the BTR and the photoreceptor belt. The BTR is rotated at the same speed as the photoreceptor so that no relative motion between the paper and the untransferred toner image occurs. The BTR consists of an aluminum core with a slightly conductive urethane rubber coating. A high bias is applied to the core which creates an electric field at the paper which causes the charged toner particles to transfer from the photoreceptor surface to the paper. The advantage of using a BTR over corona transfer is that the pressure created in the BTR nip flattens out any ripples, etc., in the paper which create gaps between the paper and the photoreceptor. These gaps decrease the strength of the field needed to transfer toner to the paper and cause deletions in the resulting copies. The same gaps can be caused by large particles, such as carrier beads or toner agglomerates from the developer housing. These create "tent" deletions which appear as white circles around the large particles. BTRs can improve the appearance of copies by greatly decreasing the diameter of the "tent" deletions.
Consequently, there is a need to clean the BTR surface because paper fibers from the backside of the copy can be attracted to the biased roll and toner which occurs on the photoreceptor between the copy regions will transfer to the roll. This toner consists of low level "background" toner, toner developed as a control patch used in maintaining the proper toner concentration and development field in the developer housing, and toner which accumulates on the lapped seam of the belt. If these materials are not cleaned from the BTR surface, they may retransfer to the back of copy sheets, appearing as spots and smudges, and if duplexed copies are being run, the spots and smudges will appear on both sides of the copies.
During development of the present invention, a device using high velocity air to remove toner and other residue from the surface of a BTR and to carry it into an evacuated chamber and then to a toner filter bag was tried. The high velocity tangential air flow was generated between two thin flexible shims and the surface of the BTR by means of a blower that evacuated the air in the cleaner housing and caused the shims to flutter. Testing revealed that, with high velocity tangential air and thin, flexible plastic shims alone, however, the cleaning results were marginal. Further testing showed that by attaching a short pile disturber fabric to the plastic shims, cleaning could be markedly improved. In this mode, the fabric material rides lightly on the surface of the roll and disturbs the toner allowing the air flowing through the fibers to move the toner into the chamber.
Several concerns exist with this arrangement, however, Abrasion of the BTR surface by the fabric may reduce roll life and the fabric material may wear because it is contacting the BTR surface. These problems may be overcome by selecting BTR and fabric materials that are not sensitive to wear, although, this option would likely be cost prohibitive in most uses currently envisioned.
Finally, the performance of the BTR air cleaner with biased shims has been predicted through testing over a range of BTR and cleaner shim biases. As known in the art, the cleaning performance necessarily depends on the charge of toner entering the transfer nip, the mass density of toner input and the efficiencies of pressure transfer and cleaning of toner by air flow alone. In particular, the biased shim BTR cleaner described herein has been shown to work over a reasonable range of currents and biases. However, additional operating latitude could be gained by increasing the cleaner air flow from the 9.5 cubic feet per minute used herein.